This is the cheapest i could find this item online for such a decent nail file. (not from my own usage) but it has over 230 reviews on amazon and its a 5 star product. Struggle to find a bad review with it

M&S are selling similar products for around £9.50AllBeatury.com is about £8

DOUBLE SIDED, SUPER DURABLE, LASTS A LIFETIME: Unlike cheap sandpaper nail files, buffers and emery boards, the fine ridges on these tools are permanently etched on both sides and never degrade no matter how many times you use it.